Improved reagent for thermal machine

ABSTRACT

The disclosed subject matter relates to a reactive matrix for the sorption/desorption of a heat transfer fluid (FG) in a reactor of a cold production device, this matrix comprising a compacted mixture of sorbent, of the manganese chloride monohydrate type, and expanded natural graphite in a preferred proportion of 79/21. The disclosed subject matter also relates to a method for manufacturing a wafer from this matrix and a reactor comprising a stack of such wafers.

CROSS-REFERENCE TO RELATED APPLICATION

This Non provisional application claims priority to PCT Application No.PCT/EP2021/079394, which was filed on Oct. 22, 2021, the entire contentsof which are incorporated by reference herein.

This invention relates to the design and manufacture of a reagent matrixused for the sorption of a cooling gas produced by the evaporation of aheat transfer fluid, or cooling fluid, in this case. Evaporation takesplace in an evaporator of a cold production machine.

Such a cold production machine comprises an evaporator and a reactorthat are arranged in a closed circuit. It uses a cooling fluid (thecoolant) which is evaporated in the evaporator and whose vapours areabsorbed by a sorbent material (the sorbent), in the reactor.

A cold production phase corresponds to the endothermic evaporation ofthe coolant in the evaporator at low pressure and to the chemicalsorption of the vapours produced by the exothermic synthesis reactionoccurring in the reactor. The heat of reaction produced must beevacuated to keep the reaction out of equilibrium and thus allow theproduction of cold to continue. The equilibrium gap is directly relatedto the rate of transformation and therefore to the thermal powerinvolved. Reaching the equilibrium conditions stops the reaction andtherefore the sorption of the cooling gas by the salt.

This phase ends when there is no more cooling fluid to be evaporated. Aregeneration phase corresponds to regeneration of the system at highpressure. By supplying heat to the reactor, at a temperature greaterthan its equilibrium temperature, the vapour is desorbed by the reagentand condenses in the condenser; the latent heat of condensation isevacuated in the surrounding environment.

The thermochemical methods can use various solid/gas working pairs:reactive salts such as oxides, hydrates, hydrides, ammoniacates. Asummary of the state of the art on the families of existing reactivepairs is provided in particular in documents “A review on adsorptionworking pairs for refrigeration,” [L. W. Wang, R. Z. Wang, and R. G.Oliveira, Renew. Sustain. Energy Rev., vol. 13, no. 3, pp. 518-534,April 2009] and “Adsorption Refrigeration Working Pairs: TheState-of-the-Art in the Application,” [A. N. Shmroukh, A. H. H. Ali, andA. K. Abel-rahman, Int. J. Chem. Nucl. Metall. Mater. Eng., vol. 7, no.11, pp. 453-465, 2013].

The choice of the reactive pair, in other words the pair formed by thecoolant and the sorbent, the implementation of the sorbent in thereactor, the operating conditions imposed and the design of the completesystem, have a direct impact on the system performance. In addition,depending on the refrigeration application targeted with this type ofmethod, the aim is to favour an implementation of the reactive materialwhich will produce either a high energy density, for a storageapplication with cold production at low power, or a high specificrefrigeration capacity, for a storage application with highinstantaneous power.

The implementation must be optimised. A high energy density promotes astorage application at the expense of a power application.

One objective of the invention is to propose a reactive pair and animplementation of the reagent that optimises both the power and thestorage capacity of a system using such a pair.

According to the invention, a reactive matrix for thesorption/desorption of a heat transfer fluid, preferably ammonia, in areactor of a cold production device, comprises a mixture of sorbent andexpanded graphite, preferably an expanded natural graphite (ENG).Preferably, the sorbent is a salt of the manganese chloride type.Advantageously, the matrix comprises a proportion of sorbent of between76% and 87%, preferably about 79%, of the mass of the matrix, with theENG forming substantially the rest of the complement to 100%. Alsoadvantageously, the matrix is compacted such that the apparent mass ofthe ENG is between 80 kg/m3 and 130 kg/m3, preferably about 120 kg/m3.Advantageously, the sorbent grain size is between 25 and 150micrometres.

According to another object of the invention, a reactive wafer for thesorption/desorption of a heat transfer fluid, preferably ammonia, in areactor of a cold production device, consists of a compact matrixaccording to the invention. Preferably, its shape is cylindrical aboutan axis and comprises a substantially coaxial interior passage.

Advantageously, such a wafer has an outer diameter of between 100 mm and110 mm, preferably equal to about 106 mm, and, the duct has a minimumdiameter of between 17 mm and 23 mm, preferably equal to about 20 mm,and, the wafer has a substantially constant axial thickness of between40 mm and 50 mm, preferably equal to about 46.5 mm.

According to another object of the invention, a method for manufacturinga wafer according to the invention, the sorbent being manganese chloridehydrate, comprises a prior drying step, preferably by heating in anoven, to obtain a manganese chloride salt of water content close to 0.1moles per mole of salt, or less than this value.

Such a method may comprise a step of mixing the manganese chloridemonohydrate with the expanded graphite in chosen proportions and a stepof compacting said mixture.

According to yet another object of the invention, a method formanufacturing a reactor comprises a step of arranging, in an envelope ofthe reactor, one or more wafers manufactured according to the invention.Such a method advantageously comprises an additional drying step, bycreating a vacuum in the envelope and, preferably, by heating thisenvelope to a temperature close to 180° C., in order to extract waterfrom the salt monohydrate, preferably until the hydration rate has beenreduced to a value close to zero moles of water per mole of manganesechloride.

The perform a compacting step, a piston is advantageously used that issubstantially cylindrical about a pistoning axis and which comprises afront surface, substantially flat and perpendicular to this pistoningaxis, and a head which extends axially from the pistoning surface, thishead comprising a substantially cylindrical distal part and a flaredconnection which connects the distal part to the front surface, thisconnection preferably forming a quarter of a circle.

Embodiments and variants will be described below, given as non-limitingexamples, and referring to the attached drawings in which:

FIG. 1 is a diagrammatic axial cross-sectional view of a reactive waferaccording to the invention;

FIG. 2 is a photograph of an expanded natural graphite (ENG) vermiculeused to form the reactive matrix for the wafer of FIG. 1 ;

FIG. 3 is a flowchart showing a method for preparing the wafer of FIG. 1;

FIG. 4 is a diagrammatic elevation view of a piston to produce a waferaccording to the invention; and,

FIG. 5 is a grain size curve for a salt used to prepare the wafer ofFIG. 1 .

FIG. 1 shows an axial cross-sectional view of a wafer 1 according to theinvention, formed from a reactive matrix 2 according to the invention.In the example shown, the wafer 1 has a cylindrical shape about an axisX, of outer diameter D7 and axial thickness E4 less than the outerdiameter D7. It comprises an axial through-passage 3 that is cylindricalabout the axis X. The passage 3 connects two opposite axial flat faces4. The passage 3 is bounded by a cylindrical inner surface 6, ofdiameter D6. The wafer 1 is bounded on the outside by a cylindricalouter surface 7, of diameter D7. The wafer has a radial thickness ERequal to (D7−D6)/2.

A reactor consists of several wafers stacked along the axis X, adjustedin an envelope 12 having walls insulating them from the outsideenvironment.

As shown on FIG. 1 , in the embodiment shown, in the cold productionphase, the evaporated fluid transits in gaseous form FG inside thepassage 3 and is absorbed along F1 by the wafer 1, through the innerwall 6. This gas FG is sorbed by the matrix 2. This sorption producesheat, evacuated along C1 through the outer surface 7 of the wafer, thenthrough the walls of this envelope, towards the outside environment.

During the regeneration phase, heat is supplied to the wafer along C2,through the walls of the envelope and of the outer surface 7. The heatsupplied causes desorption of the gas, which is released through theinner wall.

Such a system delivers a refrigeration capacity which is directlyrelated to the quality of the mass and heat transfers within the matrix2. Thus, to increase the thermal conductivity and limit itsagglomeration, the reagent is mixed with a binder to obtain aconsolidated composite material that is sufficiently porous, elastic anda very good heat conductor. The porosity is in fact important, since toensure a good mass yield of the wafer, the gas FG must spread throughoutthe matrix, over the radial thickness ER. The matrix 2 must therefore besufficiently elastic in order to withstand the significant temperatureand volume changes due to the sorption reactions. In addition, thethermal conductivity must be sufficient, since if the matrix temperatureis too high, the sorption will be slowed down or even stopped.

The binder can reduce the permeability of the matrix 2 and limit the gascirculation therein. Similarly, the swelling of the sorbent grainsduring sorption also progressively reduces the porosity.

It was decided to use a sorbent and a binder in powder form, mixed thencompacted to create a solid, anisotropic and porous matrix 2.

In the example shown, the coolant used is ammonia (NH3). The sorbent isa manganese chloride of formula MnCl₂. The form used is substantiallymanganese chloride monohydrate MnCl₂(H₂O)_(x), where x=1. The binder isan expanded natural graphite (ENG). Manganese chloride is in factparticularly hydrophilic.

Surprisingly, ENG confers certain qualities to the matrix 2:

-   -   elasticity, in order to maintain good contacts between the ENG        and the sorbent, as well as between the matrix and the walls of        the reactor envelope;    -   high thermal conductivity and low thermal mass allowing        efficient heat transfers in the matrix; and,    -   high permeability of the matrix, such that substantially all the        sorbent grains can absorb or desorb gas.

As shown on FIG. 2 in particular, the ENG generally consists ofvermicules of diameter D10; the diameter D10 is generally between 0.2and 0.4 millimetres. A vermicule may be up to 6 millimetres long.Vermicules have the shape of an accordion, a low density and a highspecific surface area. The density of raw ENG, as it is supplied, isapproximately 6 to 9 kilogrammes per cubic metre. The specific surfacearea is approximately 40 to 60 square metres per kilogramme.

The sorbent, in this case manganese chloride, consists of solid grainswhich swell or shrink depending on the step of sorbing or desorbing theammonia gas FG.

The sorbent grain size (granulometry) has a significant effect on therate of transformation and the thermal parameters of the wafer. This isdue to the interaction between the salt and the ENG which variesdepending on the size of the salt grains. The coefficient of thermalexchange by convection increases when the grain size increases; a grainsize adapted to the sorption process must therefore be chosen.

Furthermore, the very small salt grains, less than fifty microns (50μm), manage to penetrate inside the ENG vermicules, the contact surfacebetween the sorbent and the ENG is larger and the consolidated ENGmatrix is more subject to the swelling and shrinkage volume variationsof the salt grains. Nevertheless, these small grains cause the reagentto separate from the exchange surface and could be entrained by the gasflows, possibly resulting in clogging of the circuit, between theevaporator and the reactor.

It appears that the size of the grains, measured by sieving ormicro-sieving, is advantageously between twenty and one hundred andfifty microns (20 μm and 150 μm).

After producing and inserting the matrix in the envelope, a seconddrying is performed, to lower the hydration rate, preferably down tonearly zero moles of water per mole of manganese chloride.

It appears that a molecule of water in the manganese chloride evaporatesabove a temperature of between 110 and 150 degrees centigrade. Inaddition, the salt (manganese chloride) breaks down above 350 degreescentigrade. In addition, to preserve the crystalline structure of thesalt, a temperature of 200 degrees centigrade must not be exceeded. Thesalt is received as the monohydrate; it is dried in an oven at atemperature of 180 degrees centigrade plus or minus 5 degreescentigrade, until it contains only 0.1 moles of water per mole of salt.Sorbent grains not bound together can therefore be obtained.

The second drying is carried out by reducing the air pressure inside theenvelope, while heating the envelope to a temperature close to 180degrees centigrade plus or minus 5 degrees centigrade. Heating isoptional; nevertheless, this vacuuming method associated with heatingremoves the last traces of water contained in the salt. In a preferredversion, this second drying comprises the following steps:

-   -   reducing the air pressure inside the envelope, preferably at        least down to about 200 micro-bar, and heating the envelope to        about 180° C.; then,    -   allowing the reactor to cool down; then,    -   reducing the air pressure inside the envelope, preferably at        least down to about 70 micro-bar.

After performing these drying steps, the circuit is filled and thereagent is saturated with the coolant, ammonia in this case.

The grains of manganese chloride swell as sorption of the coolantproceeds, which could progressively clog the pores of the matrix andtherefore rapidly reduce the system efficiency. A matrix comprising amixture of solvent grains with a binder such as ENG limits cloggingconsiderably and maintains sufficient permeability.

According to the invention, the matrix contains a proportion by weightof about 79% of manganese chloride and 21% of ENG, the mixture beingcompacted until a density of 571 kg/m3 is obtained. Depending on theproportion of salt and ENG, the density may vary by more or less 50kg/m3. The mixture thus compacted therefore contains about 120 kg of ENGper cubic metre. Under these conditions, it appears that the matrixpermeability remains optimum, throughout the sorption/desorptionreactions.

The permeability remains sufficient when the proportion of manganesechloride is between 76% and 87% and the apparent density of the ENG,after compacting, is between 80 kg/m3 and 130 kg/m3.

In addition, in a preferred embodiment shown on FIG. 1 , the reactivematrix forms a wafer having the following dimensions:

-   -   D6 between 17 mm and 23 mm, preferably equal to about 20 mm;    -   D7 between 100 mm and 110 mm, preferably equal to about 106 mm;        and,    -   E4 between 40 mm and 50 mm, preferably equal to about 46.5 mm.

Another embodiment describes various steps of manufacturing wafers,referring to the flowchart of FIG. 3 .

The sorbent salt (MnCl₂) monohydrate is heated in an oven (step 101)until a salt having a water content of about 0.1 moles of water per moleof salt is obtained. This salt thus dried (step 102) and the ENG (step103) are then used and mixed together in the chosen proportions (step104).

After obtaining the correct mixture, a quantity of mixture required tomanufacture a wafer is measured (step 105), then mechanical compactingis carried out (step 106), to obtain the wafer 1 in its finaldimensions. Preferably, compacting is carried out axially, in otherwords in the direction of the axis X, to obtain the required axialthickness E4.

Compacting is advantageously carried out using a piston 11 of ahydraulic press 11H, for example using a piston 11 as shown on FIG. 4 ,at a pressure of between 20 and 50 bars, preferably close to 30 bars.Each wafer 1 thus produced is then evacuated from the press 11H (step106B).

The necessary steps are repeated to produce a required number of wafersto produce a reactor 13, and these wafers are arranged in the envelope12 of the reactor (step 107).

Lastly, additional drying is carried out (step 108), by creating avacuum in the envelope 12 while heating this envelope. This additionaldrying removes the water present in the envelope or in the circuit of acold production machine to which the envelope would be connected. Thisadditional drying is also carried out to extract an excess of waterwhich could have rehydrated the salt, to reduce the hydration rate to avalue preferably close to zero moles of water per mole of manganesechloride.

Another embodiment describes a preferred shape for the piston 11,referring to FIG. 4 . The shape of the wafer 1 is shown in short andlong dashes.

In the example shown, the piston 11 is substantially cylindrical about apistoning axis X11, which during the compression step 106 issubstantially coincident with the axis X of the wafer beingmanufactured. It comprises a front surface 11A, substantially flat andperpendicular to the pistoning axis X11.

The piston also comprises a head 11B which extends axially from thefront surface, intended to form the passage 3 of the wafer 1. This headcomprises a substantially cylindrical distal part. It also comprises aflared connection which connects the distal part to the front surface.This connection forms a quarter of a circle of radius R11.

Thus, as shown on FIG. 3 , the passage 3 of the wafers formed by thispiston 11 flares out at one end. This arrangement is especiallyadvantageous, since, in particular, it makes the wafer easier todemould.

Preferably, the front surface 11A and the surface of the head 11B of thepiston are given a coating that is resistant to compression,self-lubricating and non-abrasive. In the example shown, the coating isof the NEDOX SF2® type and is about 20 micrometres thick.

FIG. 5 shows a grain size curve for the salt used. Typically, a grainsize between a lower value GA, below which the salt tends to disperse inthe circuit, and an upper value GB, above which the efficiencydecreases, is chosen. Preferably, the lower value GA is close to 25micrometres and the upper limit is close to 150 micrometres. Herein, GAand GB are terms used in materials science to refers to two differenttypes of grain boundaries. GA (also known as low-angle grain boundary)refers to a boundary between two grains that are oriented at a smallangle relative to each other. GB (also known as high-angle grainboundary) refers to a boundary between two grains that are oriented at ahigh angle relative to each other.

Obviously, the invention is not limited to the examples which have justbeen described. On the contrary, the invention is defined by thefollowing claims.

It will appear to those skilled in the art that various modificationscan be made to the embodiments described above, in the light of theinformation that has just been disclosed.

Thus, in particular, but not exclusively, the dimensions of the wafersmay be different from those described previously. For example, the outerdiameter of a wafer may advantageously be between forty millimetres andtwo hundred millimetres; it may also be greater than two hundredmillimetres.

A matrix consisting of a “binder plus sorbent” composite in aconsolidated form is relatively insensitive to the mechanicalvibrations; it is therefore especially suitable for use to produce coldin a vehicle, in particular for goods transport by road.

Although manganese chloride is used in the examples describedpreviously, other types of salt can also be used. For example, calciumchloride (CaCl2)), barium chloride (BaCl2) or nickel chloride (NiCl2).Thus, different chloride/ammonia pairs can be used depending on therequirement, for example depending on whether the aim is to privilege aconstant temperature or the power of a reactor.

1. A reactive matrix for the sorption/desorption of a heat transferfluid (FG) in a reactor of a cold production device, wherein the matrixcomprises a mixture of sorbent and expanded natural graphite (ENG). 2.The matrix according to claim 1, wherein the sorbent is a salt selectedfrom manganese chloride, calcium chloride, barium chloride or nickelchloride.
 3. The matrix according to claim 1, wherein the sorbent grainsize is between 25 and 150 micrometers.
 4. The matrix according to claim1, wherein the matrix comprises a proportion of sorbent of between 76%and 87% of the mass of sorbent material, with the ENG formingsubstantially the rest of the complement.
 5. The matrix according toclaim 1, wherein the matrix is compacted such that the apparent mass ofthe ENG is between 80 kg/m³ and 130 kg/m³.
 6. A reactive waferconsisting of a compact matrix according to claim
 1. 7. The waferaccording to claim 6, wherein the wafter shape is cylindrical about anaxis (X) and in that the wafter comprises a substantially coaxialinterior passage.
 8. The wafer according to claim 7, wherein: the wafterhas an outer diameter between 40 mm and 200 mm; the passage _([ZP1]) hasa minimum diameter of between 17 mm and 23 mm; and, the wafter Chas asubstantially constant axial thickness of between 40 mm and 50 mm.
 9. Amethod for manufacturing a wafer consisting of a compact matrixcomprising a mixture of sorbent and expanded natural graphite (ENG),wherein the sorbent comprises manganese chloride hydrate, furthercomprising a prior drying to obtain a manganese chloride salt of watercontent less than 0.1 moles per mole of salt.
 10. The method accordingto claim 9, further comprising mixing the manganese chloride monohydratewith the expanded natural graphite in chosen proportions and ofcompacting said mixture.
 11. A method for manufacturing a reactor,further comprising arranging one or more wafers manufactured accordingto claim 10, in an envelope of the reactor.
 12. The method according toclaim 11, further comprising an additional drying by creating a vacuumin the envelope and by heating the envelope to a temperature close to180° C., to extract water from the salt monohydrate.
 13. The methodaccording to claim 9, wherein the compacting further comprises using apiston, that is substantially cylindrical about a pistoning axis andwhich comprises a front surface, substantially flat and perpendicular tosaid pistoning axis, and a head which extends axially from said surface,said head comprising a substantially cylindrical distal part and aflared connection which connects said distal part to the front surface,said connection forming a quarter of a circle.